380 



children of the same parents become intelligible in a simple manner 

 from the fact that each maternal germ-cell (I shall speak of 'the 

 paternal germ-cells later on) contains a peculiar combination of 

 ancestral germ-plasms, and thus also a peculiar combination of 

 hereditary tendencies. These latter by their co-operation also pro- 

 duce a different result in each case, viz. the offspring, which are 

 characterized by more or less pronounced individual peculiarities. 



But the theory which explains individual differences by referring 

 to the inequality of germ-cells, may be proved with a high degree 

 of probability by an appeal to facts of an opposite kind, viz. by 

 showing that identity between offspring only occurs when they have 

 arisen from the same egg-cell. It is well known that occasionally 

 some of the children of the same parents appear to be almost exactly 

 alike, but such children are without exception twins, and there is 

 every reason to believe that they have been derived from the same 

 egg. In other words, the two children are exactly alike because 

 they have arisen from the same egg-cell, which could of course only 

 contain a single combination of ancestral germ-plasms, and there- 

 fore of hereditary tendencies 1 . The factors which by their CO- 

 ll 1 The similar conclusion that identical ova lead to the appearance of identical 

 individuals was drawn from the same data by Francis Galton in 1875. See 'The 

 history of the Twins, as a criterion of the relative powers of Nature and Nurture,' 

 by Francis Galton, F.B.S., Journal of the Anthropological Institute, 1875, p. 391 ; 

 also by the same author, ' Short Notes on Heredity, etc. in Twins,' in the same 

 Journal, 1875, p. 325. 



The author investigated about eighty cases of close similarity between twins, and 

 was able to obtain instructive details in thirty-five of these. Of the latter there were 

 no less than seven cases ' in which both twins suffered from soine special ailment or 

 had some exceptional peculiarity ; ' in nine cases it appeared that ' both twins are 

 apt to sicken at the same time ; ' in eleven cases there was evidence for a remarkable 

 association of ideas ; in sixteen cases the tastes and dispositions were described as 

 closely similar. These points of identity are given in addition to the more super- 

 ficial indications presented by the failure of strangers or even parents to distinguish 

 between the twins. A very interesting part of the investigation was concerned with 

 the after-lives of the thirty-five twins. ' In some cases the resemblance of body and 

 mind had continued unaltered up to old age, notwithstanding very different con- 

 ditions of life,' in the other cases ' the parents ascribed such dissimilarity as there 

 was, wholly, or almost wholly, to some form of illness.' 



The conclusions of the author are as follows : ' Twins who closely resembled each 

 other in childhood and early youth, and were reared under not very dissimilar con- 

 ditions, either grow unlike through the development of natural characteristics which 

 had lain dormant at first, or else they continue their lives, keeping time like two 

 watches, hardly to be thrown out of accord except by some physical jar. Nature is 

 far stronger than nurture within the limited range that I have been careful to assign 



